In the field of petrochemical industry, gas-phase catalytic oxidation reaction using a fixed bed reactor is frequently practiced. Whereas, starting materials used in these gas-phase catalytic oxidation reactions do not necessarily have high purity.
For example, in the production of acrylic acid or methacrylic acid (hereafter collectively referred to as “(meth)acrylic acid”), first hydrocarbons are converted into unsaturated aldehydes in the first stage gas-phase catalytic oxidation step, and then in the second stage gas-phase catalytic oxidation step, the unsaturated aldehydes are converted into (meth)acrylic acid. In these reactions, normally the unsaturated aldehydes are not isolated and purified in midway the process, but the reaction gas produced in the first stage gas-phase catalytic oxidation step is introduced into the second stage gas-phase catalytic oxidation step either as it is, or after addition of molecular oxygen where necessary, to provide (meth)acrylic acid. Consequently, due to deposition and accumulation of organic substance or carbides generated from impurities contained in the starting materials of the reaction (hereafter these are collectively referred to as “catalyst inhibitor”) on the gas-phase catalytic oxidation catalyst in the first stage (“the first stage catalyst”), or due to deposition and accumulation of catalyst inhibitor generated from by-products and the like formed of the first stage reaction on the gas-phase catalytic oxidation catalyst in the second stage (“the second stage catalyst”), when these catalysts are used in the reactions continuously over a fixed period, such problems are caused as, for example, drop in the yield of the object product with time, resulting from deterioration in catalyst's performance and increase in the pressure loss at the catalyst layers.
As methods for solving such problems, for example, JP Hei 6(1994)-262081A and JP Hei 6(1994)-263689A (corres. to U.S. Pat. No. 5,442,108) disclose a method of regenerating the catalyst by regularly treating, e.g., burning, the catalyst inhibitor. More specifically, the prior art references disclose methods of safely and efficiently regenerating the catalyst by regularly suspending the reaction and heat-treating the catalyst as retaining its state of being filled in the reaction tube(s), while passing a gaseous mixture containing molecular oxygen and steam through the reaction tube(s). These methods have a merit of enabling regeneration of the catalyst, without taking it out from the reaction tube(s). However, because of the thermal load exerted on the catalyst during the high temperature treatment, the catalyst life may be shortened by every regeneration treatment depending on the kind of the catalyst. The methods that induce reduction in the catalyst life cannot be an economically satisfactory solution, and a method which enables stable continuous operation over a prolonged period is in demand.